The present invention relates to a device for measuring the torque delivery by a turbine machine and in particular a turbine engine at the input of the transmission box of a helicopter.
Helicopter engines transmit the power to the principal transmission box either at low speed through a speed reducer incorporated in the engine or at high speed which corresponds to that of the shaft line of a free turbine.
In the first case, the torque is usually measured by means of an apparatus incorporated in the speed reducer without encountering particular difficulties.
In the second case, when the rotation of a free turbine is transmitted directly, the shaft which brings the motion to the front is necessarily of great length and consequently deforms under torsion.
There is then usually employed a torque measuring device having a torsion meter which measures this torsional deformation of the shaft which is a function of the torque transmitted.
However, the torsional rigidity of the shaft is related to the value of Young's modulus of the material from which the shaft is made, and the value of Young's modulus is a function of the temperature of the shaft.
When the design of the machine so permits, the shaft is enclosed in such that the temperature is either known or may be controlled, for example by a spraying or a circulation of oil or other fluid.
The temperature of this fluid provides an acceptable image of the temperature of the shaft and can consequently be used as a correction parameter in an electronic data processing system. In the case of a shaft of great length, the latter extends through different regions in which the temperatures depend on a plurality of variables and whose evolutions are difficult to foresee during operation.
Further, if the constructional technology precludes the provision of a circulation of a fluid for homogenizing the temperature of which is the basis of the temperature of the shaft, use is made of a temperature of a static component part which may serve as a basis for the desired correction; however, sufficient precision is not obtained.
A known device employed for measuring the value of the transmitted torque comprises a first phonic or sound wheel fixed on the shaft and a second phonic or sound wheel fixed on the rear end of a shaft termed a "reference shaft" which is parallel to the first shaft, this end of the reference shaft being free while its opposite end, in the vicinity of the engine, is integral with the transmission shaft.
The angular adjustment of the wheels is effected in accordance with the operational requirements of the electronic measuring circuits employed. The initial angular offset between two homologous teeth of the wheels varies as a function of the transmitted torque.
This value of the initial setting and its increase resulting from the torque produce signals at the output of magnetic sensors disposed in the region of the phonic wheels.
This known device does not take into account variations in Young's modulus due to variations in the temperature.